The air passages constrict when allergens (flower pollen, for instance) or other irritants are inhaled.

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The episode is known as the Younger Dryas, because in Scandinavia abundant pollen from a tundra flower called the mountain avens, Dryas octopetala, reappears in soil from this date, indicating that the forest had once more given way to tundra.

WSJ: Matt Ridley on New Support for a Global-Cooling Theory | Mind & Matter

Stripes on petal veins direct bumblebees to the flower's "central landing platform" and entrance to gather nectar and pollen.

BBC: A bumblebee extracting nectar and pollen from a snapdragon